The present application is based on and claims priority from Japanese Patent Applications 2001-133876, filed May 1, 2001 and 2002-3239, filed Jan. 10, 2002, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an AC generator to be mounted in an automotive vehicle such as a passenger car or a truck.
2. Description of the Related Art
Various engine accessories mounted on a vehicle engine are generally driven by the engine via a belt. A vehicle AC generator, which is one of the engine accessories, is usually connected by a belt to the crankshaft of an engine. Therefore, if the engine rotation speed changes cyclically, the tension of the belt also changes cyclically, thereby causing vibration and noises.
In order to prevent such a problem, it has been proposed that the output current of the vehicle AC generator is switched on and off at a suitable timing. When the rotation speed of the crankshaft lowers, the output current of the AC generator is cut to reduce the torque to drive the AC generator so that the belt tension can be controlled to be constant thereby reducing the vibration or noise.
However, it is rather difficult to put the above control into practice because reliable electric parts and/or elements to be used for the above control are not available at a reasonable cost.
The output current of the AC generator also can be reduced if the time constant of the field coil of the AC generator can be reduced to a certain value. If the time constant is very small, the field current can be reduced in a short time so that the output current can be controlled in a short time. However, the field coil has a large number of turns wound around a magnetic core in order to provide necessary magnetic flux and, therefore, a large inductance. This necessarily increases the time constant, and the field current can not be changed in a short time.
Therefore, the present invention has been made in view of the above problems.
It is a main object of the invention to provide an improved AC generator for a vehicle, the time constant of which is sufficiently small.
According to a feature of the invention, a rotor of a vehicle AC generator includes a rotor core and a field coil that is comprised of a main coil and a sub-coil and a flywheel diode. The rotor core has a plurality of trapezoidal claw-pole portions that extend in opposite axial directions to interleave with each other to form a zigzag space between adjacent claw-pole portions. The main coil is disposed on a boss portion of the rotor core, and a sub-coil is series-connected to the main coil and disposed in the zigzag space. The flywheel element is connected in parallel with the main coil.
When the field current is cut off by a switching element of a voltage regulator, current that circulates through the main coil and the flywheel diode increases because the energy accumulated in the sub-coil is discharged. This increases the energy loss of the flywheel so that the circulating current disappears in a period shorter than a vehicle AC generator without the sub-coil. The flywheel element may be disposed in the rotor, so that a lead wire may not be extended from the joint of the main coil and the sub-coil. It is preferable that a resistor is connected in series with the flywheel element, because the resistor increases energy loss and decreases the life of the current that circulates through the main coil.